Lithographic offset printing plates currently employed include PS plates which are produced by using a positively working photosensitive composition mainly comprising a diazo compound and a phenolic resin or a negatively working photosensitive composition mainly comprising an acrylic monomer or a prepolymer thereof. Since all of these conventional PS plates have low sensitivity, it is necessary to conduct contact exposure from a film on which an image has already been recorded for plate-making.
On the other hand, owing to the recent technical advancements of image processing by a computer, storage of a large amount of data and data communication, input of information, revision, edition, layout, and pagination are consistently computerized, and electronic editorial system enabling instantaneous output on a remote terminal plotter through a high speed communication network or a communications satellite has been practically used. The need of the electronic editorial system has been increasing especially in the field of printing newspaper requiring immediacy. Also in the field where an original is preserved as a film from which a printing plate may be reproduced in case of necessity, it is expected that digitalized data will be stored in very large volume recording media such as optical discs.
However, few direct type printing plate precursors directly preparing printing plates based on the output from a terminal plotter have been put to practical use. For the time being, even in the field where an electronic editorial system actually works, the output is once visualized on a silver halide photographic film, which is then subjected to contact exposure to a PS plate to produce a printing plate. One reason for this is difficulty in developing a direct type printing plate precursor having high sensitivity to a light source of the plotter, e.g., an He--Ne laser or a semiconductor laser, sufficient for enabling plate-making within a practically allowable period of time.
Light-sensitive materials having high photosensitivity which may possibly provide a direct type printing plate include electrophotographic light-sensitive materials. An attempt has been made, in a system using an electrophotographic lithographic printing plate precursor in which a toner image is electrophotographically formed on an electrophotographic light-sensitive material containing photoconductive zinc oxide and then, non-image areas are subjected to oil-desensitization with an oil-desensitizing solution to obtain a lithographic printing plate, to apply a light-sensitive material having high sensitivity to semiconductor laser beam to the electrophotographic light-sensitive material.
For example, the use of specific spectral sensitizing dye is proposed as described, for example, in JP-B-2-28143 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-63-124054 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-63-241561, and JP-A-63-264763. Further, improvements in a binder resin for a photoconductive layer are proposed in order to increase photosensitivity and to reduce background stains in non-image areas (i.e., to improve water retentivity of non-image areas) as described, for example, in JP-A-63-220148, JP-A-1-116643, and JP-A-2-69759.
Since these plate-making techniques are based on oil-desensitization of zinc oxide for making it hydrophilic, and a specific oil-desensitizing solution and specific dampening water are used, there are various restrictions in that color inks usable are limited, in that printing durability is markedly reduced when neutral paper is employed as printing paper, and in that a printing machine in which a plate of this kind and a PS plate are exchangeably used must be thoroughly cleaned.
It is also known to electrophotographically make a lithographic printing plate by removing a photoconductive layer of non-image areas after the toner image formation. Printing plate precursors suitable for use in such a system are described, for example, in JP-B-37-17162, JP-B-38-6961, JP-B-38-7758, JP-B-41-2426, JP-B-46-39405, JP-A-50-19509, JP-A-50-19510, JP-A-52-2437, JP-A-54-145538, JP-A-54-134632, JP-A-55-105254, JP-A-55-153948, JP-A-55-161250, JP-A-57-147656, and JP-A-57-161863.
In order to use an electrophotographic light-sensitive material as a printing plate, binder resins which can be dissolved or swollen with an alkaline solvent and thereby removed are often used in the photoconductive layer so that the photoconductive layer in non-image areas can be etched with an alkaline etchant to expose the underlying hydrophilic surface. The resins soluble or swellable in the alkaline solvent are usually less compatible with organic photoconductive compounds than polycarbonate resins widely employed as binder resins for electrophotographic light-sensitive materials. Accordingly, the amount of the organic photoconductive compound to be incorporated into a photoconductive layer is limited. When a content of the organic photoconductive compound in a photoconductive layer is low, a transfer rate of carrier in the photoconductive layer is reduced even if a sufficient amount of carrier for offsetting the surface potential is generated in the photoconductive layer and, as a result, a rate of surface potential decay, i.e., a rate of response is reduced. This means prolongation of the time after exposure required for the surface potential to decay to a sufficient level for causing no fog and for starting toner development. As an exposure illuminance increases in order to shorten the exposure time for the purpose of minimizing the processing time, the above-described response time becomes longer. Therefore, the slow response is a great hindrance to achievement of reduction in total processing time.
Scanning exposure with a light source of high illuminance, e.g., a laser light source, arouses another problem. Specifically, if the response is slow, since the rate of surface potential decay differs between the area where scanning has started and the area where scanning ends, the resulting image suffers from fog in the latter area, although free from fog in the former area. This is disadvantageous for plate-making.
Binder resins which have conventionally been used in electrophotographic lithographic printing plate precursors include styrene-maleic anhydride copolymers, vinyl acetate-crotonic acid copolymers, vinyl acetate-maleic anhydride copolymers, and phenolic resins as described, for example, in JP-B-41-2426, JP-B-37-17162, JP-B-38-6961, JP-A-52-2437, JP-A-54-19803, JP-A-54-134632, JP-A-55-105254, JP-A-50-19509, and JP-A-50-19510.
It has been pointed out, however, that these known binder resins have various disadvantages when they are used in electrophotographic lithographic printing plate precursors using an organic photoconductive compound. For example, when a styrene-maleic anhydride copolymer resin is used as a binder resin, the film formed is rigid and may cause cracks in case of bending the plate. Also, the layer is so poor in adhesion that the plate fails to withstand mass printing. A film formed by using a phenolic resin as a binder resin is brittle and has poor printing durability. A film of a vinyl acetate-crotonic acid copolymer or a vinyl acetate-maleic anhydride copolymer also exhibits poor printing durability. In addition, satisfactory electrophotographic characteristics, especially charge retention in dark and photosensitivity cannot be secured with any of these resins.
Copolymers comprising an acrylic ester or methacrylic ester and a carboxylic acid-containing monomer are described in order to solve the above-described problems in JP-A-57-161863 and JP-A-58-76843. These binder resins make it feasible to use an electrophotographic light-sensitive material as a printing plate precursor. Nevertheless, the recently posed problem arising from the slow response described above, i.e., insufficient photosensitivity, still remains unsolved.
Further, in JP-B-1-209458 copolymers comprising an acrylic ester or methacrylic ester containing an aromatic ring and an acid group-containing monomer, e.g., a carboxylic acid are described, for achieving improved printing durability and photosensitivity. However, while the performance properties described above may be improved, these copolymers are disadvantageous in that the photoconductive layer of non-image areas (areas other than toner image areas) is not easily and rapidly removable so that strict control of conditions for removal is required.
More specifically, the problem in that the conditions for achieving complete removal of only non-image areas without causing dissolution of even minute toner image areas thereby to produce a printing plate having a reproduced image with high fidelity and causing no background stains are restricted is still unsolved.
In addition, in the above-described system in which the whole photoconductive layer of the non-image areas is dissolved out in an alkaline processing solution, the dissolved material is accumulated in the alkaline processing solution. Therefore, when the processing solution is used for successive treatment of a large number of plate precursors, problems, for example, precipitation of agglomerates and reduction of the dissolving power may occur.